1. Field of the Invention
The present invention relates to a molding material feed screw used in a molding material supply device of an injection molding machine.
2. Description of the Related Art
A molding material supply device is arranged between a material supply port and a hopper or hopper loader of an injection molding machine in order to supply material to the injection molding machine. During molding by an injection molding machine, a molding material from the hopper or hopper loader, and mainly a resin in the form of pellets, is supplied to the injection molding machine via a molding material supply device while being suitably cut into desired portions.
The relationship between the molding material supply device and the injection molding machine body is explained using FIG. 8.
In FIG. 8, a molding material supply port 23 of a molding material supply device 20 is connected to a hopper 22, and a material supply port 14 of an injection molding machine body 10 is connected to a material discharge port 25 of the molding material supply device 20. A molding material feed screw 24 driven by a driving device such as a motor 21 is rotatably arranged within the molding material supply device 20. This molding material feed screw 24 has spiral grooves formed therein by providing a screw flight 40.
The injection molding machine body 10 is provided with a heating cylinder 11 housing a plasticizing screw 15 for filling molten resin into a metal mold clamped by a clamping device (not shown). The heating cylinder 11 is installed with a heater 13 around the outer periphery thereof for heating the resin supplied to the heating cylinder 11 by the molding material, supply device 20.
Resin pellets 30 (see FIG. 9) used as molding material supplied from the hopper 22 to the molding material supply port 23 of the molding material supply device 20 are transported to the material discharge port 25 through grooves of the molding material feed screw 24 by rotation of the molding material feed screw 24 driven by the motor 21, transferred to the material supply port 14 of the injection molding machine body 10, and used for molding. The resin pellets can be suitably apportioned into amounts required for molding according to the amount of rotation of the molding material, feed screw 24 of the molding material supply device 20.
Normally, the molding material supply port 23 of the molding material supply device 20 is filled with the resin pellets 30, and the resin pellets 30 are fed inside the molding material supply device 20 while being pushed by the spiral screw flight 40 provided on the molding material feed screw 24 due to rotation of the molding material feed screw 24. In order to ensure stable transport of the resin pellets 30, the gap between the outer periphery of the screw flight 40 provided on the molding material feed screw 24 and the cylindrical inner wall 28 of a cylindrical space 27 of the molding material supply device 20 is preferably as small as possible.
However, if the outer diameter of the screw flight 40 approaches the inner diameter of the cylindrical space 27 in order to reduce the size of the gap between the outer periphery of the screw flight 40 and the cylindrical inner wall 28 of the molding material supply device 20, a phenomenon occurs in which the resin pellets 30 become entrapped between the edge (material supply port edge 26) of the molding material supply port 23 of the molding material supply device 20 and the outer edge of the screw flight 40 during rotation of the molding material feed screw 24.
FIG. 9 depicts a resin pellet 31 entrapped between the material supply port edge 26 and the outer periphery of the screw flight 40. Furthermore, the material supply port edge 26 serves as the connection between the molding material supply port 23 and the cylindrical space 27 (see FIG. 8).
If the resin pellets 30 become entrapped between the material supply port edge 26 and the outer edge of the screw flight 40, a considerable load acts on the rotating operation of the molding material feed screw 24, and a large force (torque) is required to drive and rotate the molding material feed screw 24. In addition, crushing of the entrapped pellet 31 or scraping along the cylindrical inner wall 28 by the entrapped pellet 31 can cause defective molding. Moreover, a considerable load may act continuously on rotation of the molding material feed screw 24 as a result of the entrapped pellet 31 being pushed into the gap between the cylindrical inner wall 28 and the outer periphery of the screw flight 40.
In order to avoid this entrapment of the resin pellets 30, entrapment of resin pellets was conventionally suppressed by forming a notch in the outer periphery of the screw flight 40 in a region of the molding material feed screw 24 of the molding material supply device 20 where entrapment occurs.
For example, Examined Japanese Utility Model Publication No. 62-37693 discloses a technology of a raw material processing device installed with a screw feeder in which a gap with the inner diameter of a raw material supply unit was partially increased in size by cutting out a portion of a screw flight near the front edge of a raw material supply port.
FIG. 10 is an explanatory drawing of the related art in which a notch is provided in the outer periphery of the screw flight 40 in the molding material supply device 20.
As shown in FIG. 10, the outer diameter of the screw flight of a region of the screw flight 40 where entrapment of the resin pellets 30 occurs is made to be somewhat smaller than the outer diameter of the screw flight 40 at other portions. In other words, the outer diameter of a notched portion 43 of the screw flight 40 is smaller than the outer diameter of a non-notched portion 42. In addition, a boundary 41 of the notch where the outer periphery of the screw flight 40 has been cut out is parallel to an axis of rotation 59 of the molding material feed screw 24. As a result of increasing the size of the gap between the screw flight 40 and the cylindrical inner wall 28 of the region where entrapment occurs as shown in FIG. 10, entrapment of the resin pellet 30 can be reduced.
However, when a notch and large gap are provided in a portion of the screw flight 40, the force that feeds the resin pellets 30 in the forward direction of the molding material supply device 20 ends up decreasing. In particular, since the portion of the level difference formed in the boundary 41 of the notch of the portion where the notch is provided in the screw flight 40 is formed parallel to the feed screw axis of rotation 59, a phenomenon occurs in which the resin pellets 30 continue to rotate together with the molding material feed screw 24 as a result of being pushed into that portion containing the level difference.
If the resin pellets 30 continue to rotate together with rotation of the molding material feed screw 24, since the resin pellets 30 wear down and become thin due to friction of the resin pellets 30 or the resin pellets 30 soften due to heat of friction, the resin pellets 30 end up becoming entrapped in the gap between the outer periphery of the screw flight 40 and the inner wall of the cylindrical space 27 of the molding material supply device 20. As a result, a large load is imparted to rotation of the molding material feed screw 24 or the formation of foreign objects occurs during molding.